High surface area alumina solid

ABSTRACT

An alumina solid, obtainable by a process comprising the step of contacting in a liquid medium at least one alumina precursor with at least one template comprising a dendrimeric molecular nanosystem a mixture of two or more thereof.

The present invention relates to a high specific surface area alumina solid, to a process for preparing it and to the use of particular molecular nanosystems as templates.

The preparation of alumina solids is known per se. Processes currently in industrial use are surveyed in Ullmann's Encycl. Tech. Chem., 5th ed. 1997, pp. 561-562, and in Winnacker, Kuchler, 4th edition, 1983 vol. 3, pp. 2-41.

Alumina thus prepared has a specific surface area in the range from 100 to 400 m²/g and pore diameters of from 2 to 5 nm, with large-pore aluminas generally having smaller surface areas. Such large-pore aluminas have a broad pore size distribution up to 10 nm.

Mesoporous oxides have likewise been described, for example in DE-A 44 07 326 and DE-A 195 43 638. According to these references, mesoporous oxides are prepared by adding a cationic, anionic or nonionic surfactant as a structure-directing reagent, or template, to the oxide precursors (monomers or oligomers) in the course of the polycondensation process to give an inorganic oxide. According to these references, the actual synthesis is followed by thermal removal of the template surfactants, for example by calcining in air at from 350 to 600° C., giving rise to a mesoporous, purely inorganic oxide.

“Mesoporous” means that the diameter of the pores in the solid is from about 2 to about 50 mn.

The pores of prior art mesoporous oxides are predominantly confined to the range around 2 nm or so.

However, oxide solids which have larger pores and which are inexpensive to produce are desirable for some applications, for example for use as heterogeneous catalysts, to optimize mass transfer in the reaction. This aim is achieved with some of the commercially available aluminas which, however, have broad pore diameter distributions and small specific surface areas. Furthermore, heat treatment may lead to the formation of various crystalline alumina phases (eg. γ-Al₂O₃ or, at even higher temperatures, α-Al₂O₃) some of which have inherent catalytic properties such as Lewis or Broensted acidity and therefore have an undesirable effect on the reaction to be catalyzed.

It is an object of the present invention to provide an alumina solid which is suitable for a multiplicity of applications, in particular for use as catalyst or catalyst support, and a process for preparing this solid in an economically favorable, i.e. inexpensive, way.

We have found that, surprisingly, this object is achieved by using a dendrimeric molecular nanosystem as template in the preparation of the alumina solid.

The present invention accordingly provides a process for preparing an alumina solid, which comprises the step of contacting in a liquid medium at least one alumina precursor with at least one template comprising a dendrimeric molecular nanosystem or a mixture of two or more thereof.

This invention further provides an alumina solid obtainable by a process comprising the step of contacting in a liquid medium at least one alumina precursor with at least one template comprising a dendrimeric molecular nanosystem or a mixture of two or more thereof.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an exemplary schematic illustration of the synthesis of the dendrimeric molecular nanosystems used herein.

FIG. 2 shows the result of nitrogen adsorption on the solid of Example 1. Here, TMS stands for trimethylsilyl. M-i represents a monomer, C-i a core, W-i a monodendron, and D-i a dendrimer. A filled circle  stands for a benzene ring, for a 1,3,5-linked benzene ring, and for a 1,4-linked phenylene unit. The symbol is a 1,3-di-tertbutyl-phenyl unit. A line—stands for a C≡C connection between benzene rings.

FIG. 3 shows the X-ray diffraction pattern of the solid of Example 1. Here, p/p⁰ is the relative pressure, N₂ ^(ad) is the volume of nitrogen, adsorbed at the solid, + represents a measured value registered during adsorption, ++ a measured value registered during desorption.

FIGS. 3a and 3 b show enlargements of sections of this X-ray diffraction pattern. Here, I is the intensity of the registered signal in count units.

“Template” refers to a substance which is initially mixed with the starting materials and subsequently removed from the resulting solid, eg. by thermal treatment, to leave the areas of the solid in which the template was located in the form of pores.

The term “dendrimeric molecular nanosystem” as used herein refers to molecules which are branched similar to the branches of a tree. They usually have a central core, a defined number of generations and (functional) terminal groups and are preferably prepared by a reaction sequence that is repeated.

As stated above, templates used according to the invention are dendrimeric molecular nanosystems. Preference is given to dendrimeric molecular nanosystems containing, as branching point, nitrogen, phosphorus, carbon or a mixture of two or more thereof. A selection of dendrimeric molecular nanosystems particularly preferably used according to the invention is given by Xu and Moore in Angew. Chemie 105 (1993) 1394-1396, by

Vögtle et al. in Angew. Chem. Int. Ed. Engl. 31 (1992) 1571-1576, by D. Brabandervan den Berg and Meijer in Angew. Chemie 105 (1993) 1370-1372, by Tomalia et al. in Angew. Chem. Int. Ed. Engl. 29 (1990) 138-175, and in WO 96/15097, which are all fully incorporated herein by reference.

Examples for dendrimeric molecular nanosystems which may be used in the present invention are:

phenylacetylene dendrimers, preferably those whose solubility is improved by using peripheral solubility-improving groups, for example di-tert-butylphenyl groups; dendrimers in the form of phosphonium and polyammonium salts containing tris(p-methoxymethylphenyl)phosphine and triethanolamine as monomers; arborols comprising crown ethers, especially those having diaza-18-crown-6 units in addition to aromatic units;

arborols on the basis of saturated hydrocarbons as described, for example, by Newkome et al. in Angew. Chem. 103 (1991) 1205-1207;

polypropylene imine dendrimers, polyethyleneimine dendrimers or dendrimers composed of a combination of propylene imine and ethylene imine units, especially those comprising a diaminobutane core and terminal CN or NH₂ groups. Further examples are:

polyamidoamine dendrimers,

polylysines,

poly(ethyleneimine) dendrimers,

hydrocarbon dendrimers (iptycenes),

polyether dendrimers, such as those based on polyerythritol,

polythioether dendrimers,

polyamide dendrimers (Denkewalter dendrimers),

polyamidoalcohol dendrimers (arborols),

polyarylamine dendrimers, described in the Tomalia et al. reference cited above, which is fully incorporated herein by reference.

The term “branching point” used in the definition of the nanosystems used according to the invention refers to an atom where the structure of the molecular nanosystem branches out.

For further explanation we attach FIG. 1 which is an exemplary schematic illustration of the synthesis of the nanosystems used according to the invention taken from the Xu and Moore reference cited above.

The amount of nanosystem used is generally not subject to any particular restrictions and is preferably from about 0.05 to about 50% by weight, more preferably from about 2 to about 10% by weight, in each case based on the synthesis batch, ie. the total amount of alumina precursor, template and liquid medium.

Liquid media used in the process of the invention are preferably aqueous solutions, for example mixtures of water and alcohols, eg. ethanol and/or isopropanol. However, contacting may also be carried out in water, an organic solvent or a mixture of different organic solvents. Preference is given to using alcohol solvents generated by hydrolysis of the alumina precursors, ie. ethanol when using aluminum ethoxide (Al(OEt)₃), isopropanol when using aluminum isopropoxide and butanol when using aluminum butoxide.

A usable alumina precursor is any compound which is converted to alumina by calcining in air at elevated temperatures. The precursor may be employed, for example, as an organometallic component, eg. as alkoxide, Grignard compound, alkylate, chelate, eg. with acetylacetonate, in a form which is soluble in an organic solvent, or in the form of soluble salts, as a hydroxide, as a colloid in an aqueous phase or as a combination of two or more thereof.

Contacting can be carried out at a basic, acidic or also at a neutral pH, preference being given to an acidic pH, especially a pH of from 1 to 5.

After combining the alumina precursor and the template in a liquid medium, the resulting suspension is brought to the appropriate pH and stirred at preferably from about 10 to about 150° C., more preferably from about 10 to about 90° C., especially from about 20 to about 65° C., for from about 0.5 to about 72 hours, preferably from about 1 to about 48 hours, especially from about 10 to about 30 hours. When contacting the components described above, the pressure is preferably from about 0.4 to about 300 bar, more preferably from about 0.8 to about 150 bar and especially from about 1 to about 10 bar.

The resulting suspension is then separated from the liquid medium, for example by centrifugation or simple filtration, and subsequently dried. Drying is preferably effected initially at ambient temperature for from about 5 to about 72 hours, preferably from about 10 to about 48 hours, especially from about 20 to about 30 hours, and then at an elevated temperature in the range of from about 50 to about 100° C., preferably from about 55 to about 70° C., for several hours.

The alumina solid dried in this manner is subsequently calcined at from about 350 to about 800° C., preferably from about 400 to about 700° C., especially from about 450 to about 600° C., for from about 2 to about 10 hours, preferably from about 4 to about 6 hours, in the presence of oxygen, preferably in air.

During the contacting of the alumina precursor with the template, the following additional components may be added:

pharmacologically active organic or inorganic compounds such as analgesics or cardiovascular agents, in which case the incorporation of these compounds into the alumina solid results in retarded release of the pharmacologically active compound on application;

enzymes for biotechnological application such as oxidases, reductases, transferases, hydrolases, lyases, isomerases, ligases and semi-synthetic and synthetic enzymes as described, for example, in Science, 223 (1984) p. 165ff, Cold Spring Harbor Symp. Quant Biol. 52 (1987), p. 75-81, and Tetrahedron 40 (1994), p. 269-292, the relevant contents of which are fully incorporated herein by reference;

pigments such as ferromagnetic pigments, eg. chromium(IV) dioxide, ferrites, iron oxides, iron or iron alloys, further inorganic pigments, eg. chalk, graphite, titanium white, white lead or zinc white, carbon black, luminescent pigments, eg. zinc sulfide or alkaline earth metal aluminates, organic pigments such as azo pigments, indigoid pigments, phthalocyanine pigments, metal complex pigments or diketopyrrolopyrrole pigments, which are likewise present in encapsulated form in the resulting alumina solid.

In the preparation of the alumina solid of the invention, it is also possible to add, in particular via the aqueous phase, an ionic compound of an element of main groups I to III except aluminum or of transition groups I to VIII of the Periodic Table of the Elements, the lanthanoids, silicon, germanium, tin, lead, phosphorus, antimony, bismuth, sulfur, selenium, tellurium or a mixture of two or more thereof, preferably sodium, potassium, calcium, magnesium, beryllium, boron, gallium, indium, silicon, germanium, tin, lead, antimony, bismuth, scandium, yttrium, lanthanum, titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, manganese, rhenium, iron, cobalt, nickel, ruthenium, copper, zinc, cadmium, mercury, cerium, europium, thorium, uranium or a mixture of two or more thereof, more preferably sodium, potassium, calcium, magnesium, beryllium, boron, gallium, indium, silicon, germanium, tin, lead, bismuth, titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, manganese, rhenium, iron, cobalt, nickel, ruthenium, copper, zinc, cadmium, mercury, cerium or a mixture of two or more thereof.

The elements cited above may be employed especially in the form of their sulfates, phosphates, nitrates, carbonates, halides and perchlorates and also in the form of easily hydrolyzable organometallic compounds such as alcoholates, chelates, carboxylates, preferably in the form of sulfates, phosphates and nitrates, more preferably in the form of sulfates and nitrates.

They can also be employed in the form of isopoly cations or heteropoly cations as described in references DE-A 44 07 326 and DE-A 195 43 638 which were mentioned at the beginning and the relevant contents of which are likewise fully incorporated herein by reference.

In a further embodiment of the present invention, the mixture of alumina precursor, template and liquid medium may be applied to an inert porous support, for example a porous glass, a silica gel, diatomaceous earth or clay, a ceramic material, a metal, a metal packing or a metal mesh as used for static mixers or reactive distillation column packings, for example. This makes it possible to produce inert supports covered or impregnated with alumina solid by the process of the invention to provide composite materials which have improved mechanical stability and improved permeability in the catalytic reactor compared to the starting materials.

In this context, metallic support materials, for example the stainless steels having the material numbers 1.4767, 1.4401, 2.4610, 1.4765, 1.4847, 1.4301, etc., are particulary useful since their surface can be roughened by heat treatment before they are covered/impregnated with the solids. Particular preference is given to using Kanthal (material number 1.4767) or alumina-containing metals as mesh material. Kanthal is an alloy containing about 75% by weight of Fe, about 20% by weight of Cr and about 5% by weight of Al. Heat treatment is effected by heating the metallic supports cited above in air at from 600 to 1100° C., preferably from 800 to 1000° C., for from one to twenty hours, preferably from one to ten hours, and cooling back down. This pretreatment is described in EP-A-0 564 830 and is important since the heat treatment significantly improves the bonding between the solids and the metallic supports.

If contacting is effected at a stationary interface, ie. at the interface of two immiscible fluids, the alumina solid may be obtained in the form of thin films or layers since the presence of such an interface makes it possible to provide the solid, during its preparation, with a particular oriented structure such as a thin film. Further details are described in DE-A 196 24 862. Thin films or layers obtained in this manner may be employed in membrane, separation or purification processes or may be used for information storage.

Such applications are described, for example, in DE-A 44 24 221, the relevant contents of which are fully incorporated herein by reference.

Such solids may be utilized in particular for electronic, optical or electro-optic applications; corresponding membranes are utilized in the catalytic conversion in membrane reactors or in reactive distillations. The present invention accordingly also provides for the use of a solid of the invention as catalyst or catalyst support, as matrix for active components, in membrane, separation or purification processes, for producing electrical, optical or electro-optic components such as switching devices or sensors, for producing oxide ceramics or for separating substances, as adsorbent, as filler, especially in polymers, as fire retardant and as abrasives and polishing materials.

If the solid of the invention is used as catalyst or catalyst support, it is particularly useful in hydrocarbon oxyfunctionalization, olefin oxidation to give oxiranes, aromatics alkylation, hydrogenation, dehydrogenation, hydration, dehydration, isomerization, an addition reaction, an elimination reaction, nucleophilic or electrophilic substitution, dehydrocyclization, hydroxylation of heteroaromatic compounds, aromatics hydroxylation, epoxy-aldehyde rearrangement, amination of monomeric or oligomeric olefins, an aldol type condensation reaction, a polymerization reaction, an esterification or etherification, the catalytic conversion of exhaust gases and flue gases and for nitrogen oxide removal.

The solid of the invention is in particular characterized by the following properties:

X-ray analysis, its strongest reflections are in the angular range of less than 4° (2 Θ) when measured using Cu K_(α)radiation.

The solid has pores in the range from about 2 to about 50 nm, preferably from about 2 to about 30 nm, especially from about 3 to about 10 nm, the pore diameter being determined by nitrogen adsorption at 77 K. The specific surface area determined under the same conditions as evaluated by the method of Barret, Joyner and Halenda according to J. Am. Chem. Soc., 73 (1951), 373-380, is more than about 100 m²/g, preferably more than 250 m²/g, especially from about 300 to 550 m²/g.

The pore volume, likewise determined by nitrogen adsorption at 77 K, is more than about 0.1 ml/g, preferably more than about 0.25 ml/g, especially from about 0.25 ml/g to 1.0 ml/g.

The present invention further provides the use of a dendrimeric molecular nanosystem or a mixture of two or more thereof as templates in the preparation of an alumina solid.

The Examples which follow illustrate the preparation and the properties of the solid of the invention and relate it to a Comparative Example prepared according to a prior art method, ie. without adding a dendrimeric molecular nanosystem.

EXAMPLE 1

A solution of 205.8 g of aluminum triisopropoxide (Merck), 300 g of ethanol and 81.0 g of isopropanol was homogenized for 30 minutes in a 2 l four-neck flask. 50.0 g of dendrimeric N₁₄ amine (N,N′-bis-(3-aminopropyl)-ethylenediamine: BASF AG) were then added to this mixture. Upon addition of a solution of 650.0 g of deionized water and 7.5 g of hydrochloric acid solution (10% by weight) a white suspension was formed. The white suspension was stirred at room temperature for 24 h, filtered and dried in air for 24 h. The solid was then dried at 60° C. overnight. 95.3 g of solid were obtained. The solid was finally calcined in air at 500° C. for 5 hours. The loss on calcination was 77% by weight, based on the solid used.

Nitrogen adsorption at 77 K gave a typical hysteresis in the relative pressure range p/p°>0.45 as shown in FIG. 2. Application of the BJH model to these results gave a pore surface area of 450 m²/g for pores in the range from 3 to 4 nm. The corresponding pore volume was 0.42 ml/g as determined at a relative pressure p/p° of 0.98. The most frequent pore diameter was about 3.7 nm.

X-ray analysis of the solid obtained in this manner gave the highest reflection intensities at angles of less than 4° (2 Θ) (Cu K_(α)radiation). The diffraction pattern is shown in FIGS. 3, 3 a and 3 b.

COMPARATIVE EXAMPLE 1

A solution of 205.8 g of aluminum triisopropoxide (Merck), 300 g of ethanol and 51.0 g of isopropanol was homogenized for 30 minutes in a 2 l four-neck flask. Upon addition of a solution of 650.0 g of water and 7.5 g of the hydrochloric acid solution (10% by weight) a white suspension was formed. The white suspension was stirred at room temperature for 24 h, filtered and dried at room temperature in air for 24 h and then at 80° C. overnight. 87.7 g of solid were obtained. The solid was then calcined at 500° C. for 5 hours. The loss on calcination was 28.2% by weight.

X-ray analysis of the solid obtained in this manner gave no distinct reflections at angles of less than 4° (2 Θ). The solid was pure γ-Al₂O₃.

Nitrogen adsorption at 77 K gave typical hystereses in the relative pressure range p/p°>0.4. Application of the BJH model to these results gave pore sizes in the range from 2 to 5 nm. The pore surface area according to the BET method was 289 m²/g. The corresponding pore volume was only 0.37 ml/g as determined at a relative pressure p/p° of 0.98. The most frequent pore diameter was about 3.8 nm. 

We claim:
 1. A process for preparing an alumina solid, which comprises the step of contacting in a liquid medium at least one alumina precursor with at least one template comprising a dendrimeric molecular nanosystem or a mixture of two or more thereof.
 2. The process defined in claim 1, wherein the dendrimeric molecular nanosystem contains, as branching point, a nitrogen atom, a phosphorus atom, a carbon atom or a mixture of two or more thereof.
 3. The process defined in claim 1, wherein the alumina solid comprises at least one element selected from the group consisting of sodium, potassium, calcium, magnesium, beryllium, boron, gallium, indium, silicon, germanium, tin, lead, bismuth, titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, manganese, rhenium, iron, cobalt, nickel, ruthenium, copper, zinc, cadmium, mercury and cerium.
 4. The process defined in claim 1, further comprising contacting the precursor with a pharmacologically active organic or inorganic compound, an enzyme, a pigment or a mixture of two or more thereof.
 5. The process defined in claim 1, wherein the contacting is carried out in the presence of an inert porous support or at a stationary interface.
 6. An alumina solid obtainable by a process comprising the step of contacting in a liquid medium at least one alumina precursor with at least one template comprising a dendrimeric molecular nanosystem or a mixture of two or more thereof. 